The demand for hydrogen has been increasing day-by-day for the hydro-treating processes in petroleum industries and also for hydrogen fuel cells, both stationary and non-stationary fuel cells. Since hydrogen is a non-polluting fuel, its use as a fuel, particularly for fuel cells, has been increasing very fast. However, the well-established proton exchanged membrane fuel cells require carbon monoxide-free hydrogen as a fuel to avoid deactivation of the noble metal catalyst in the fuel cells.
The main natural sources of hydrogen are hydrocarbons and water. Among the hydrocarbons, methane has the highest hydrogen to carbon ratio and hence it is the most preferred choice among the hydrocarbons for hydrogen production.
The conventional processes for the production of hydrogen are based on steam reforming of hydrocarbons, such as naphtha and methane or natural gas, and auto thermal reforming of hydrocarbons, particularly heavier hydrocarbons. The hydrogen production processes have been recently reviewed by Fierro and coworkers [reference: Pena, M. A.; Gomez, J. P. and Fierro, J. L. G.; Applied Catalysis A: General; Volume 144, pages 7 to 57, year 1996].
Both the hydrocarbon steam reforming and auto thermal reforming processes are operated at high temperatures, above about 900° C., and the product stream of these processes contains appreciable amounts of carbon monoxide along with hydrogen. The removal of carbon monoxide at low concentrations from hydrogen is very costly. Hence, the hydrocarbon steam reforming and auto thermal reforming processes are not economical for the production of carbon monoxide-free hydrogen. Thus, there is a practical need to develop a process for the production of hydrogen from methane as methane has the highest hydrogen to carbon ratio among the hydrocarbons at temperatures lower than that used in the conventional hydrocarbon steam reforming and auto thermal reforming processes.
Production of Carbon Monoxide-Free Hydrogen From Methane:
A few processes are known also for the production of carbon monoxide-free hydrogen from methane.
Recently, Kikuchi [reference: Kikuchi, E.; Hydrogen-permselective membrane reactors, CATTECH, March 1997, pages 67 to 74, Baltzer Science Publishers] has described a process based on steam reforming of methane in a membrane reactor to produce hydrogen free of carbon monoxide. By applying a Pd/ceramic composite membrane to steam reforming of methane over a commercial supported nickel catalyst, methane conversion up to 100 percent can be accomplished in a Pd-membrane reactor at temperatures as low as 500° C. to produce carbon monoxide-free hydrogen. In this process, the hydrogen produced in the steam reforming of methane is continuously removed from the reaction system by the selective permeation of hydrogen through the Pd-membrane. However this process has not yet been commercialized and it has the following drawbacks/limitations: (1) because of the use of a number of Pd-membrane tubes, the capital cost of this process is very high; (2) there is a possibility that the Pd-membrane becomes deactivated by deposition of carbonaceous matter; and (3) there is also a problem of membrane stability and/or a possibility of membrane failure due to formation of pinholes in the membrane.
A Japanese patent [JP 09234372 A2, Sep. 09, 1997] discloses a process for the manufacture of hydrogen by thermal decomposition of hydrocarbons at 200° C. to 1000° C. using a catalyst containing nickel, alkali or alkaline earth compounds. A Russian patent [RU 2071932 C1, Jan. 20, 1997] discloses the production of hydrogen and carbon by thermal decomposition of methane on nickel catalyst.
A recent Japanese patent [JP 11228102 A2, Aug. 24, 1999] discloses reactors for thermal decomposition of methane to form carbon and hydrogen. Hydrogen production by catalytic cracking of methane or natural gas and other hydrocarbons, at below 900° C. using nickel-based catalyst, is disclosed in a few publications [reference: Zhang, T. and Amiridis, M. D., Applied Catalysis A: General, Volume 167, pages 161 to 172, year 1998; Muradov, N. Z., Energy Fuels, Volume 12, pages 41 to 48, year 1998; Kuvshinov, G. G. et. al.; Hydrogen Energy Progress XI Proceedings of World Hydrogen Energy Conference, 11th, Volume 1, pages 655 to 660, Edited by Veziroglu, T., year 1996; and Muradov, N. Z., Proceedings of US DOE Hydrogen Program Review, Volume 1, pages 513 to 535, year 1996].
In the above prior art processes, based on catalytic cracking or thermo-catalytic decomposition of methane or other hydrocarbons, the hydrogen produced is free from carbon monoxide and carbon dioxide, but the catalyst deactivation is fast due to the carbon formed on the catalyst and this is accompanied with a fast increase in the pressure drop across the catalyst bed, making the process unpractical for the hydrogen production.
Recently, Choudhary and Goodman reported a process for the production of carbon monoxide-free hydrogen involving stepwise methane steam reforming [reference: Choudhary, T. V. and Goodman, D. W., Catalysis Letter, Volume 59, pages 93 to 94, year 1999]. In this process, methane pulse and water pulses are passed over a pre-reduced nickel-based catalyst at 375° C., alternatively. When the methane pulse is passed over the catalyst, the methane from the pulse is decomposed to hydrogen and carbon, leaving the carbon deposited on the catalyst according to the reaction:CH4→C+2H2↑  (1)
When the water pulse is passed over the catalyst with the carbon deposited on it, the carbon on the catalyst reacts with steam to form CO2 and hydrogen according to the reaction:C+2H2O→CO2+2H2  (2)In some cases, the products of the above reaction are also accompanied by an amount of unreacted methane.
In this process, although the carbon monoxide-free hydrogen is produced by catalytic cracking of methane and the carbon deposited on the catalyst is removed by the cyclic operation of the methane and water pulses in the same reactor, the process is not operated in the steady state and the hydrogen produced is not continuous. Hence, it is not practical and also not economical to produce carbon monoxide-free hydrogen on a large scale by this transient process involving cyclic operation of methane and water pulses.
Very recently, Choudhary et al. have reported a possibility of the continuous production of hydrogen at 500° C. by carrying out the above two reactions, Reactions 1 and 2, simultaneously, in two parallel catalytic reactors in a cyclic manner by switching a methane containing feed, 18.2 mole percent CH4 in N2, and a steam containing feed, 20.5 percent steam in N2, between the two reactors at predecided intervals of time, and combining the product streams of the two reactors [reference: V. R. Choudhary, S. Banerjee and A. M Rajput, Journal of Catalysis, Volume 198, page 136, year 2001]. However, both the reactions, Reactions 1 and 2, are thermodynamically favored at higher temperatures. The methane decomposition, Reaction 1, is also favored at lower pressure or lower concentration of methane. Our preliminary studies show that both the methane conversion in Reaction 1 and the degree of carbon gasification in Reaction 2 are decreased sharply with increasing the methane concentration and for decreasing the temperature. Hence, using undiluted or less diluted methane, as a feed, permits the very high cost of separation of the diluent to be reduced. For obtaining high conversion of methane, Reactions 1 and 2 need to be carried out at a higher temperature, above about 600° C. However, at such a high temperature, a significant amount of carbon monoxide is formed in Reaction 2 and therefore carbon monoxide-free hydrogen cannot be obtained by the above cyclic process.
Because of the above-mentioned drawbacks and limitations of all the prior art processes, there is a great need for developing a process for the continuous production of carbon monoxide-free hydrogen by catalytic decomposition of methane or natural gas at a temperature below about 900° C., while avoiding the carbon build-up on the catalyst by its time-to-time removal by some means.